The present invention provides an improved coring bit motor and a method for obtaining a material core sample from the side wall of a drilled well.
Wells are generally drilled into the earth""s crust to recover natural deposits of hydrocarbons and other desirable and naturally occurring materials trapped in geological formations. A slender well is drilled into the ground and directed to the targeted geological location from a drilling rig at the surface. In conventional xe2x80x9crotary drillingxe2x80x9d operations, the drilling rig rotates a drillstring comprised of tubular joints of steel drill pipe connected together to turn a bottom hole assembly (BHA) and a drill bit that are connected to the lower end of the drillstring. During drilling operations, a drilling fluid, commonly referred to as drilling mud, is pumped and circulated down the interior of the drillpipe, through the BHA and the drill bit, and back to the surface in the annulus.
Coring is generally a process of removing an inner portion of a material by cutting with an instrument. While some softer materials may be cored by forcing a coring sleeve translationally into the material, for example soil or an apple, harder materials generally require cutting with rotary coring bits; that is, hollow cylindrical bits with cutting teeth disposed about the circumferential cutting end of the bit. The cored material is generally captured within the coring apparatus for retrieval from the well bore. Coring is typically used to remove unwanted portions of a material or to obtain a representative sample of the material for analysis to obtain information about its physical properties. Coring is extensively used to determine the physical properties of downhole geologic formations encountered in mineral or petroleum exploration and development.
Conventional coring of wells drilled to recover naturally occurring hydrocarbons is performed using a coring bit and core barrel attached to the end of the drill string. The core is captured inside the core barrel as the rotating coring bit penetrates the formation of interest. This coring process substantially disrupts the normal drilling process because the drill bit has to be removed from the end of the drill string and replaced with a coring bit. Coring in this manner can be very time consuming and costly. However, this method usually provides for a high rate of success for obtaining core samples for all of the formation drilled through in this manner.
Conventlonal side wall rotary coring is characterized by the use of a coring bit with a hollow, cylindrical configuration and cutting teeth embedded about the circumference of one open end. The coring bit is generally rotated about its axis as it is forced against the side wall of the well. As a core sample is cut from the side wall, the core sample is received into the hollow barrel defined by the interior walls of the coring bit. The optimal speed of rotation of the coring bit and the optimal weight on bit (the magnitude of the axial force urging the bit into the side wall) are generally determined by the type of formation being cored and by the physical characteristics of the coring bit.
Petroleum and other naturally occurring deposits of minerals or gas often reside in porous geologic formations deep in the Earth""s crust. A formation of interest in a drilled well can be investigated using a coring tool to obtain representative samples of rock taken from the wall of the well adjacent to the formation of interest. The representative rock sample is generally cored from the formation using a hollow, cylindrical coring bit. Rock samples obtained through side wall coring are generally referred to as xe2x80x9ccore samples.xe2x80x9d Core samples are physically removed from the wall of the well and retrieved within the coring tool to be transported to the surface.
Analysis and study of core samples enables engineers and geologists to assess important formation parameters such as the reservoir storage capacity (porosity), the flow potential (permeability) of the rock that makes up the formation, the composition of the recoverable hydrocarbons or minerals that reside in the formation, and the irreducible water saturation level of the rock. These estimates are crucial to subsequent design and implementation of the well completion program that enables production of selected formations and zones that are determined to be economically attractive based on the data obtained from the core sample.
Several coring tools and methods of obtaining core samples have been used for conventional side wall coring. There are generally two types of coring methods and apparatus, namely rotary coring and percussion coring. The present invention is directed towards rotary coring, the more preferred method because of the quality of the core sample obtained.
Rotary coring of side walls generally involves forcing an open and exposed circumferential cutting end of a hollow cylindrical coring bit against the wall of the well and rotating the coring bit to promote cutting at the leading end. The coring tool is secured against the wall of the well at the zone or formation of interest with the rotary core bit oriented towards the wall of the well. The coring bit is deployed radially outwardly away from the coring tool axis and toward the wall of the well.
The coring bit is generally coupled to a coring motor through an extendable shaft or mechanical linkage. The shaft or linkage advances the rotating coring bit axially towards the side wall to bring the cutting end of the coring bit into contact with the side wall. The coring bit penetrates into the side wall by removing rock within a cylindrical cutting zone. The circumferential cutting end of the coring bit has a plurality of teeth and is often embedded with carbides, diamonds or other materials with superior hardness for cutting rock.
A cylindrically-shaped core sample is received into the hollow interior of the coring bit as cutting of the core sample progresses. After a core sample of the desired length is received, the core sample is broken free from the formation rock by breaking the remaining connection (radial cross-section) within the open, cutting end of the coring bit. The core bit and the core sample within it are retrieved into the coring tool by retracting the shaft or linkage used to extend the coring bit to its deployed position. The retrieved core sample may be ejected from the coring bit within the coring tool to allow use of the coring bit for obtaining subsequent samples at the same or different depths.
Rotary coring is the preferred method of obtaining a core sample because the core sample retains its flow and storage properties without the fracturing and compaction involved in percussion coring. However, efficient rotary coring requires efficient use of limited space. Because of the number of components and the physical manipulations required to recover a conventional side wall core sample, conventional rotary side wall coring presents many challenges associated with the limited space available downhole. As wells are successfully being drilled to deeper formations, and as directional wellbores reach further and further from the true vertical location of the surface location, these wells necessarily become more slender, thereby providing less space for positioning, deploying and operating conventional coring devices.
While it is favorable to obtain as large a representative sample as can be had from the side wall, there are physical limitations that make obtaining a larger core sample difficult and costly. The length of the core sample is limited by the stroke or travel of the coring bit. That is, from the time the cutting teeth of the coring bit initially touch the side wall, the maximum axial displacement into the side wall is determined by the mechanical characteristics of the coring tool.
The mechanical configuration of prior art coring tools is dictated by several different parameters. For cutting, the rotary coring bit must be rotated on its axis using some portable source of mechanical power contained within the coring tool. Motors that turn the core bit in coring tools are typically hydraulic motors driven by high pressure oil provided by an electrically powered pump. The electrically powered hydraulic oil pump is powered by electricity provided to the motor through the conductive cable that is used to lower, raise, control and to generally position the coring tool within the wellbore. Rotation of the coring bit is typically obtained by coupling the coring bit to the hydraulic motor using a mechanical linkage. Furthermore, upon deployment the coring bit must be extended from within the coring tool housing outwardly to the external side wall, and then further extended into the side wall during rotation of the coring bit to cut the core sample. Finally, after cutting of the core sample is completed, the coring bit and the core sample contained therein must be retracted to within the coring tool. If other subsequent core samples are to be obtained using the same coring bit, the core sample must be ejected from the coring bit and stored within the coring tool for transport to the surface. All of the mechanical devices, the hydraulic motor, the mechanical linkage from the motor to the coring bit for rotating and extending the bit, and the coring bit itself, must be xe2x80x9cstoredxe2x80x9d in their inactive configuration within the slender coring tool housing until the tool is in position adjacent to the zone of interest in the side wall. When used, the coring tool must provide the needed rotation, as well as the extension and retraction of the coring bit in order to successfully obtain the core sample. The physical and dimensional challenges are substantial, and the present invention provides a more efficient and compact device and method for obtaining the core sample.
Additionally coring devices in the prior art are generally very mechanically complex and as such are prone to a wide variety of failures during operation, making them highly unreliable in the downhole environment. As a result, many oil companies are reluctant to use them due to the often poor success rate in recovering side wall core samples.
What is needed is a device that can extend and apply force through the coring bit against the side wall, retract the coring bit to within the coring tool after the core sample is obtained, and turn the coring bit at a desirable angular velocity throughout the process of cutting the core sample. What is needed is a device that can extend, retract and rotate the coring bit without complex mechanical linkages that take up valuable space, i.e. an efficiently xe2x80x9cpackagedxe2x80x9d device that, when in the inactive, undeployed position, takes up little space within the coring tool. What is needed is an improved coring motor that is sufficiently compact that two or more coring motors can be used in a single coring tool to obtain multiple samples.
The present invention provides a solution to the problem of side wall conventional coring in the limited-space environment of slender wellbores. The retrieval and analysis of core samples in their undamaged condition provides valuable geologic information that drastically improves analysis and decision-making on the part of the oil company geologist.
The present invention provides an improved coring motor that is actually two motors, a spin motor and a thrust motor, working together to control the rotation, weight-on-bit and extension or retraction of the coring bit. The spin motor is comprised of a spin stator, a spin rotor and a spin rotor sleeve. The thrust motor is similarly comprised of a thrust stator, a thrust rotor and a thrust rotor sleeve. These two motors are each coupled to a specially designed drive shaft that is connectable at its end to a coring bit. The drive shaft is designed to rotate by operation of the spin motor and to extend and retract by operation of the thrust motor. The extension of the drive shaft and coring bit toward the side wall, and the subsequent retraction of the drive shaft and coring bit back to within the coring tool, are effected by varying the speed of the thrust motor relative to the speed of the spin motor. This design allows extremely efficient packaging of one or more of the improved coring motors within a single downhole coring tool.